Use of laser spectroscopy to measure the 13C/12C and 18O/16O compositions of carbonate minerals

The stable carbon and oxygen isotope compositions of carbonate minerals are utilized throughout the earth and environmental sciences for various purposes. Here, we demonstrate the first application of a prototype instrument, based on off-axis integrated cavity output laser spectroscopy, to measure the carbon and oxygen isotope composition of CO₂ gas evolved from the acidification of carbonate minerals. The carbon and oxygen isotope ratios were recorded from absorption spectra of ¹²C¹⁶O¹⁶O, ¹³C¹⁶O¹⁶O, and ¹²C¹⁶O¹⁸O in the near-infrared wavelength region. The instrument was calibrated using CaCO₃ minerals with known δ¹³C_VPDB and δ¹⁸O_VSMOW values, which had been previously calibrated by isotope ratio mass spectrometry relative to the international isotopic standards NBS 18 and NBS 19. Individual analyses are demonstrated to have internal precision (1 SE) of better than 0.15‰ for δ¹³C and 0.6‰ for δ¹⁸O. Analysis of four carbonate standards of known isotopic composition over 2 months, determined using the original instrumental calibration, indicates that analyses are accurate to better than 0.5‰ for both δ¹³C and δ¹⁸O without application of standard−sample−standard corrections.